In recent years, wireless data communication in domestic and enterprise environments have become increasingly commonplace and an increasing number of wireless communication systems have been designed and deployed. In particular, the use of wireless networking has become prevalent and wireless network standards such as IEEE 801.11a (Trademark) and IEEE 801.11g (Trademark) have become commonplace.
The requirement for increasing data rates, communication capacity and quality of service has led to continued research and new techniques and standards being developed for wireless networking. As a specific example, the Institute of Electrical and Electronic Engineers (IEEE) have formed a committee for standardising a high-speed WLAN standard known as IEEE 802.11n (TradeMark). The 802.11n™ standard comprises Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications aimed at enhancing WLANs to provide higher effective data throughputs. IEEE 802.11n™ is expected to operate in the 5 GHz frequency spectrum and promises data rates of around 100 Mbps and above on top of the MAC layer. 802.11n™ will use many techniques which are similar to the earlier developed IEEE 801.11a™ and IEEE 801.11g™ standards. The standard is to a large extent compatible with many of the characteristics of the earlier standards thereby allowing reuse of techniques and circuitry developed for these. For example, as in the previous standards IEEE 801.11a™ and IEEE 801.11g™, 802.11n™ will use Orthogonal Frequency Division Multiplex (OFDM) modulation for transmission over the air interface.
In order to acquire e.g. synchronisation data for a signal, it is known to transmit known data. The receiver may determine characteristics of the received signal and the propagation channel by evaluating the distortions introduced to the known data by the transmission.
Specifically, in IEEE 801.11a™ and IEEE 801.11g™ a number of sub-carriers of each OFDM symbol is allocated for transmission of known pilot symbols and each OFDM symbol thus comprises a pilot tone comprising four pilot symbols. In IEEE 801.11a™ and IEEE 801.11g™, the pilot tone comprises the pilot symbols [1 −1 1 1] or [−1 1 −1 −1] in subcarriers #−21, #−7, #7 and #21 respectively following the definitions in the IEEE802.11a standard.
In the current 802.11n™ proposals there are several pilot sets proposed: 2 or 4 pilot tones for 20 MHz bandwidth modes, 4 or 6 or 8 pilot tones for 40 MHz bandwidth modes.
A disadvantage associated with OFDM is that the transmitted signal has very large amplitude variations in the time domain. In particular the Peak to Average Power Ratio (PAPR) is typically large and this large dynamic range requires that the power amplifier has a large linear dynamic range as any non-linearity will result in a degraded performance. In more detail, the OFDM time domain samples typically follow a Rayleigh distribution approximately and can attain very high peak amplitude.
In order to deal with these high peak amplitudes the power amplifier is typically substantially backed-off from the non-linear operating region. However, this results in a reduced dynamic range for a given supply voltage and results in increased complexity, cost and power consumption for the power amplifier.
In order to reduce this disadvantage, a number of techniques have been developed to reduce PAPR for OFDM signals. Such techniques include using some subcarriers for selection of dummy data symbols that result in a reduced PAPR, increasing the symbol constellation points and selection between these to reduce PAPR, and addition of a time-domain sequence chosen from a predefined set and communication of the chosen sequence to the receiver. However, most of these techniques are unsuitable for communication systems such as IEEE 802.11 systems because they would require substantial system modifications, additional signalling and/or be contrary to the technical standards. For example, most of these techniques would be incompatible with the defined standards, algorithms and techniques already developed for IEEE 802.11 systems.
An example of a PAPR reduction scheme is presented in “Joint channel estimation and peak-to-average power reduction in coherent OFDM: a novel approach” by Fernandez-Getino Garcia, M. J.; Edfors, O.; Paez-Borrallo, J. M.; Vehicular Technology Conference, 2001. VTC 2001 Spring. IEEE VTS 53rd, Volume: 2, 6-9 May 2001, Pages: 815-819 vol. 2. The described technique comprises selecting pilot symbols of an OFDM signal such that the time domain PAPR is reduced. Furthermore, specific OFDM sub carriers are reserved for transmitting information to the receiver of which pilot signals have been selected.
However, such an approach is incompatible with many communication systems where the use of a specific set of pilot symbols is prescribed e.g. by the standards for the communication system. In particular, the described approach is incompatible with IEEE 802.11 communication systems. Furthermore, the increased signalling of information reduces the effective throughput and efficiency of the communication network.
“Orthogonal pilot sequences for peak-to-average power reduction in OFDM” by Fernandez-Getino Garcia, M. J.; Paez-Borrallo, J. M.; Edfors, O.; Vehicular Technology Conference, 2001. VTC 2001 Fall. IEEE VTS 54th, Volume: 2, 7-11 Oct. 2001, Pages: 650-654 vol. 2 discloses a similar approach relying on the use of orthogonal pilot sequences to enable a receiver to determine the selected pilot symbols by blind detection thereby obviating the necessity of transmitting information of the selected pilot symbols. However, the described approach is incompatible with many communication systems, such as IEEE 802.11 communication systems, which do not allow a free selection of pilot symbols or the use of orthogonal pilot sequences.
Hence, an improved system would be advantageous and in particular a system allowing increased flexibility, improved performance, increased throughput, reduced signalling; suitability for a large range of pilot sequences and/or improved compatibility with existing systems and techniques and in particular with IEEE802.11 communication systems would be advantageous.